Digital subscriber line (DSL) technology provides for digital data transmission over the twisted pair wires of a local telephone network. A DSL network utilizes the available frequency spectrum on the twisted pair wires to carry digital signals between a customer's DSL modem and a central office (the “local loop”). Each DSL modem transmits within an assigned frequency band having a number of channels. Each channel is characterized by its power spectral density (PSD) and its transfer bit-rate.
In the local loop, groups of twisted pair wires are brought together and housed in a protective jacket or sheath, referred to as a binder. Because several wires are brought in close proximity to one another within the binder, capacitive coupling between channels from different modem bands results in the leakage of signal power from one channel into another, causing one signal to affect another signal. The result is “crosstalk” and it is generally, but not always, worse between adjacent pairs in the sheath.
A modem's transmit spectrum includes the power spectral density (PSD) for each channel in the band. Among other factors, the transmit spectrum determines the distribution of crosstalk from the modern's line into the lines of other modems. Consequently, the shape of a modem's transmit spectrum determines the rate of successful data transfer for the transmitting modem and (potentially) causes crosstalk into the channels of other modem(s), thereby lowering the rate of successful data transfer of the other modem(s). Modems are programmed to respond to crosstalk by swapping bit-rates between channels. However, it is not uncommon to have crosstalk interference on a majority of a modem's channels, making it improbable that the modem is going to achieve its target data rate.
Transmit spectrum management includes identifying or predicting the channels within a binder that are (or will be) affected by a modem's signal. Three considerations of transmit spectrum management are (1) a DSL binder has many lines, and consequently many channels (e.g., 40,000), (2) only a relatively small number of those lines (and channels) are in “crosstalk proximity” to one another, and (3) line attributes such as length and gauge factor into a line's susceptibility to crosstalk interference. Transmit spectrum management techniques may also consider line attenuation, which causes a decrease in the PSD of the line's channels. Longer lines are especially susceptible to data-rate transfer drop-off due to crosstalk. Goals of transmit spectrum management include establishing a transmit spectrum for a modem band such that the transmitting modem achieves its target data-rate while minimizing crosstalk into susceptible and higher bit-rate channels of other modem bands.
An existing approach for reducing crosstalk interference includes reducing the transmission power for the signal(s) that are causing crosstalk, and increasing the transmission power for the signal(s) that have low data transfer rates until all lines have the same received data rate. However, this approach does not provide for achieving a desired data rate on a line while minimizing crosstalk into a channel on a second line.